S. R. Faruqi:

dahar dahar jalnā = to burn with a sound/voice [āvāz]

The expression dahar dahar jalnā is an exceedingly fresh and effective image. Ahmad Mushtaq too has used that image very well, and it's possible that he might have seen it in Mir and then used it:

āg to chāroñ or lagī hai pattī pattī bhaṛak rahī hai
dahaṛ dahaṛ jaltī haiñ shāḳheñ dekhūñ aur gużartā jāʾūñ

[fire has caught in all four directions, every leaf is flaring up,
the branches burn audibly/cracklingly; I would look and keep on passing by]

This is what's called 'taking creative advantage'-- not a crude/uncouth imitation in the style of Firaq Sahib! With regard to mood, and to freshness of words, this verse recalls Mir's

{124,2}.

Both are very fine verses.

Mir has deliberately and poetically written dahar dahar ; and with the warrant [sanad] of this very verse, [the dictionaries] farhang-e āṣifiyah and nūr ul-luġhāt have entered dahar dahar jalnā as an idiom. In reality the idiom is dhaṛ dhaṛ jalnā or dahaṛ dahaṛ jalnā (Platts). Ahmad Mushtaq has versified it correctly, and Mir, with regard to the wordplay of the rhyme, has changed the retroflex ṛ [rā-e hindī]. Up to the first half of the eighteenth century, poets used to make use of such freedoms. A verse of Sauda's is noted below, but this opening-verse of his makes the matter entirely clear [because in it an ordinary re is rhymed with a retroflex ṛe]:

sāqī sīmīñ tirī shab dekh kar gorī gorī
sharm se shamʿa huʾī jātī hai thoṛī thoṛī

[Cupbearer, having seen by night your bright, white silveriness,
from shame the candle keeps becoming less and less]

From Jamia Millia, Dr. ʿabd ul-rashīd has informed me that the idiom can also be dahr dahr jalnā , with the scansion of = - = - [fāʿilāt], as is proved by the following verses. By Mir Sajjad:

hotī nahīñ hai sard hamāre yih dil kī āg
lāgī hai jis zamāne se jaltī hai dahr dahr

[it doesn't cool down, this fire in our heart,
from the time when it caught, it has burned audibly/cracklingly]

By Sauda:

ṭhīk hotī hai jis ghaṛī dopahar
lage hai dahr dahr jalne dahar

[it's fine, at whatever time afternoon comes,
my fate has begun to burn audibly/cracklingly [like a time signal]]

It's surprising that all the dictionaries have turned out to be devoid of this idiom. Dr. 'Abd ul-Rashid has also pointed out to me a verse by aḥsan ul-dīn bayān :

mashshad-e parvānah raushan kyūñ nah hove dahr dahr
jis kī bālīñ par tamām-e shab khaṛī rotī hai shamʿa

[why wouldn't the tomb of the Moth be illumined dahr dahr
at whose headboard all night the candle stands weeping?]

But in my opinion here dahr dahr has the meaning of 'at all times', because the idiom is dahar dahar jalnā , not dahar dahar raushan honā .

FWP:

SETS == EK; IDIOMS
MOTIFS
NAMES
TERMS == FRESH WORD

Although I usually go for painstakingly literal translation, thā vuh bhī ik zamānah is so colloquial that to render it as 'even/also that was a single/particular/unique/excellent era' just seems absurd. Something like 'those were the days' (which similarly can't be captured in English by taking it as a mere literal identification of which days are under discussion) seems much more to the point. There's a tone of nostalgic relish that comes with the idiom.

Probably the original idiom dhaṛ dhaṛ jalnā was onomotopoetic, and through those great old Indic retroflex sounds it echoed the crackling and popping of a vigorous fire. In any case, as SRF notes, it certainly works effectively as a 'fresh word'.

The speaker seems to be nostalgic for those days, as he wistfully recalls a time when his now-burnt-out laments had such a fiery power that-- that what? That they consumed the wilderness for miles around in all directions? That they were 'effective' in getting a response of some kind? That they evoked, if not the beloved's voice, then at least the audible 'voice' of the flames?